Modernize codebase by replacing NULL with nullptr

Fixes -Wzero-as-null-pointer-constant warning.

Test: m
Bug: 68236239
Change-Id: I5b4123bc6709641315120a191e36cc57541349b2
diff --git a/libc/bionic/futimens.cpp b/libc/bionic/futimens.cpp
index 1ca8eb5..03f7e38 100644
--- a/libc/bionic/futimens.cpp
+++ b/libc/bionic/futimens.cpp
@@ -30,5 +30,5 @@
 #include <sys/stat.h>
 
 int futimens(int fd, const struct timespec times[2]) {
-  return utimensat(fd, NULL, times, 0);
+  return utimensat(fd, nullptr, times, 0);
 }